Effects of ultrasonic, microwave, and combined ultrasonic-microwave pretreatments on the enzymatic hydrolysis process and protein hydrolysate properties obtained from Chinese sturgeon (Acipenser sinensis)

Degree of hydrolysis (DH), yield, amino acid profile, protein solubility, and antioxidant activity of Chinese sturgeon protein hydrolysates, as influenced by thermal pretreatment, ultrasonic (US), microwave (MW), and combined US-microwave pretreatments were investigated. Initially, the samples were subjected to thermal pretreatments in order to measure their effect on DH, which increased at 55 degrees C. The DH recorded 7.63, 5.55, and 6.02% for US, MW, and combined US-MW pretreatment (US + MW), respectively, at the optimal time (8 min). The enzymatic hydrolysis (EN) of pretreated samples increased the DH to 19.41, 14.18, and 16.91% for US + EN, MW + EN, and US + MW + EN, respectively. The US + EN treatment was most effective for obtaining higher DH and yield, which were 19.41% and 18.62%, respectively. The use of US + EN also resulted in an increase in the percentage of molecular weights (<= 1,000 Da), amino acid content and protein solubility, which reached 89.24, 80.08, and 98.58%, respectively. While, US + MW + EN pretreatment has achieved the highest antioxidant activities by IC(50)of 1,1-diphenyl-2-picrylhydrazyl and 2,2-Azinobis (3-ehtylbenzothiazoli- 6-sulfnic acid), which were 3.01 and 1.85 mg/ml, respectively, in addition to the reducing power assay, which was 0.528 at a protein concentration of 5 mg/ml. Therefore, US and combined US-MW techniques can play a promising role in the production of protein hydrolysates and the improvement of their antioxidant properties. Practical applications Nowadays, interest in Chinese sturgeon production has increased as a promising source of protein and antioxidant peptides. The optimal thermal pretreatment can be used to enhance the degree of hydrolysis. The results indicated that the use of ultrasound as a pretreatment enhanced the degree of hydrolysis, which could be useful in the preparation of protein hydrolysate with higher yields. The use of combined US-MW significantly improved the antioxidant properties of the protein hydrolysate. The combined US-MW technique is a novel method for obtaining valuable peptides and protein hydrolysates that can be applied as antioxidant constituents in the food products.